The “Rayleigh diffraction limit”, which is the spatial resolution with which an object can be detected, is limited by the wavelength of the radiation used for detection. Higher frequencies are therefore required to resolve smaller objects.
Microwaves of high frequency are absorbed in the atmosphere at rates exponentially higher than microwaves of low frequency. Accordingly, low frequency radar is preferred for longer range. However, due to the Rayleigh diffraction limit, the ability to distinguish two objects adjacent to each other, referred to as “resolving power”, is proportional to the ratio of wave length to aperture. As a result, for a unit aperture, radar can only distinguish an object if the wavelength of the electromagnetic radiation is the same or smaller than the object. The Rayleigh diffraction limit combined with the earth atmosphere's attenuation profile forces radar designers to choose between long range at low resolution, or short range at high resolution. In an extreme example, penetrating radars such as foliage penetrating radar (FOPEN) or ground penetrating radar (GPR) require low frequencies to minimize attenuation within the penetrated medium. Consequently, only the very largest objects can be resolved, diminishing the utility of such radar systems.
Conventional ocean SOund NAvigation and Ranging (SONAR) sensor systems can be either passive or active. Passive SONAR is restricted to receiving signals, while active SONAR both transmits and receives signals. Active SONAR operates by transmitting a beam of sound waves through water. Sound waves travel faster through water than through air, and more rapidly through salt water than through fresh water. Target detection occurs when this beam encounters an object having different density than that of the medium through which the SONAR beam is being transmitted (sea water). The beam then bounces off the target and may be detected by receivers positioned to receive the reflected beam.
Active SONAR is extremely useful in that it gives the exact position of a target, but it also has some significant drawbacks. Active SONAR is noisy, and can be easily detected through passive SONAR near the emitting SONAR. Furthermore, the resolution characteristics of active SONAR do not allow for the exact identification of the target. In addition, active SONAR may operate at frequencies over which marine life is sensitive.
Ultrasound systems provide images of muscles, tendons, and many internal organs, their size, structure and any pathological lesions with real time tomographic images. Ultrasound is commonly used to visualize a fetus during routine and emergency prenatal care. Although generally believed to be safer than ionizing radiation, some of the known effects of ultrasonic energy are enhanced inflammatory response and soft tissue heating. Thus, there is some concern that prolonged exposure to ultrasonic energy can affect tissue health and development.
According to concepts of quantum mechanics, a quantum system may exist in several states simultaneously corresponding to different values of a physical observable such as position, momentum, or spin. Changes among properties of entangled quanta can be correlated. The composite system is described by a state, that is, a topological structure of substates describing specific observables. Each of these states corresponds to eigenvalues of some set of observables (e.g., quanta positions). In quantum entanglement, the quantum states of two or more quanta are described with reference to each other, even though the individual objects may be spatially separated.